1. Field of the Invention
The present invention relates to data communications, and more particularly, to adaptive receiver techniques using decision updating.
2. Description of the Related Art
Much of the public switched telecommunications network (PSTN) is implemented using digital data transport. Nonetheless, significant portions of the PSTN are still based on analog technology. For example, the “local loop” portion of PSTN that connects a telephone subscriber to a central office (CO) is typically an analog loop.
A current generation of 56 Kbps modems no longer assumes that both ends of a communications path may be analog and suffer impairment due to quantization noise introduced by analog-to-digital converters (ADCs). Instead, such modems are designed to exploit configurations in which there is only one analog portion in a downstream transmission path from a digitally connected server modem to a client modem connected to an analog local loop. This assumption is reasonable in areas where most Internet Service Providers (ISPs) and business customers are digitally connected to the network and allows data signaling rates of up to 56 Kbps in the downstream transmission path.
Although a variety of similar designs are available, modems conforming to the ITU-T Recommendation V.90 are illustrative. See generally, ITU-T Recommendation V.90, A Digital Modem and Analogue Modem Pair for Use on the Public Switched Telephone Network (PSTN) at Data Signalling Rates of up to 56 000 Bit/S Downstream and up to 33,600 Bit/S Upstream (09/98). Recommendation V.90 defines a method for signaling between a modem connected to an analog loop (the analog modem) and a modem connected to the digital trunk (the digital modem). Modems in accordance with Recommendation V.90 take advantage of this particular arrangement to increase the data signaling rate from the digital modem towards the analog modem. In particular, detrimental effects of quantization noise can be avoided if there are no analog-to-digital conversions in the downstream path from the digital V.90 modem to the analog modem. In such cases, the PCM codes from the digital modem are converted to discrete analog voltage levels in the local CO and are sent to the analog modem via the analog local loop. The analog modem's receiver then reconstructs the discrete network PCM codes from the analog signals received.
Using current techniques, 56 Kbps signaling rates can be achieved. However, signaling rates may be limited by distortion introduced in the digital backbone itself. One source of distortion is Robbed Bit Signaling (RBS). RBS is an in-band signaling technique used in some portions of the PSTN to perform control functions such as conveyance of ring and call progress indications in the telephone network. In short, RBS involves modification by the PSTN of data transmitted thereover. In particular, a least significant bit (LSB) of certain PCM codewords may be used (or usurped) by a portion of the digital backbone. Though RBS is generally acceptable when codewords carry a voice signal, RBS effectively acts as noise or distortion and may limit the information carrying capacity of a communications channel that includes a portion employing it.
Recognizing these limitations, techniques have been developed for detecting, characterizing and mitigating RBS. For example, U.S. Pat. No. 5,875,229 to Eyubolglu et al. proposes detection and characterization of RBS during a “training” phase prior to other training operations such as initialization of equalizer coefficients. Eyubolglu's characterization technique is based on counting LSB values equal to logic zero and logic one in each of 24 intervals of a received training signal. U.S. Pat. No. 5,859,872 to Townshend also proposes a scheme in which RBS is detected during an initial training phase. In Townshend, a decoder first attempts to equalize a received training signal having a known pattern by minimizing the difference between its output and the known pattern under the assumption that no bit robbing has occurred. The decoder then measures the average equalized values at each of six phases and determines for each phase which of 4 bit robbing schemes (including no bit robbing) has been employed. Once the bit robbing that occurred in each phase is determined, the equalization process is rerun, since the first equalization was performed without knowledge of the bit robbing.